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Exadata Hybrid Columnar Compression (EHCC) FAQ (Doc ID 1910687.1)

Last updated on JANUARY 21, 2020

Applies to:

Exadata Database Machine V2 - Version All Versions to All Versions [Release All Releases]
Information in this document applies to any platform.

Purpose

 This document addresses the frequently asked questions related to Exadata Hybrid Columnar Compression.

Questions and Answers

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In this Document
Purpose
Questions and Answers
 What are the types of compression available with EHCC ?
 Can CTAS be used to create EHCC tables ?
 How to convert existing / non-EHCC table to use EHCC ?
 How to disable EHCC compression on a table ?
 Can we use DBMS_REDEFINITION online redefinition package for EHCC tables ?
 How does update works for EHCC table ?
 What is Compression Unit?
 How to check compression format for a particular row ?
 How to get estimate of compression ratios for EHCC compression types ?
 How to enable tracing for EHCC ?
 How to query stats about CUs ?
 What storage types are supported for EHCC or HCC ?
 Can we specify different compression type for each partition in a table ?
 Does EHCC support function based indexes ?
 Is EHCC suitable for single row access methods ?
 Does EHCC uncompress CUs to access the data? Will this result in multiple I/Os for a single row access ? 
 Can we use EHCC on normal server with SUN ZFS filesystem ?
 What are supported RDBMS version for EHCC ?
 If table is already OLTP compression on, will I be able to enable EHCC on the same table ?
 Does EHCC supports BLOB data type ?
 Does EHCC supports LONG data type ?
 Can both EHCC and OLTP compression will be on for a single table ?
 In a warehouse with rows being added to an EHCC table via ETL, can the new rows be converted to EHCC, or the whole table to be periodically recompressed ?
 Are there any application code changes if we convert tables to EHCC ?
 Can we join a regular table with EHCC table in a query ?
 Which view to check the compression type ?
 What's the difference between query low and query high ?
 Does Oracle Golden Gate supports EHCC from source to target replication ?
 If one table is marked as query high, can it be moved to archive high or any other EHCC method directly ?
 What is the impact on performance if taking a table out of EHCC? specially if the table is large ?
 In EHCC we may have a table to have mix of HCC and OLTP compression. Does this create extra overhead with fetching data when range scan is used ?
 Is the row migrated out of CU when update occurs causing row migration ?
 Default segment size is 8MB, can EHCC compression make it less than 8MB ?
References

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